1. Field of the Invention
The present invention relates to wireless telecommunications and, more particularly, to methods and systems for determining which sector or sectors in a cellular wireless network should serve a given mobile station. The invention conveniently uses the location of the mobile station as a basis to select one or more sectors that should be considered members of the mobile station's “active” set of sectors.
2. Description of Related Art
Cellular wireless is an increasingly popular means of personal communication in the modern world. People are using cellular wireless networks for the exchange of voice and data over cellular telephones, Personal Digital Assistants (“PDAs”), cellular telephone modems, and other devices. In principle, a user can seek information over the Internet or call anyone over a Public Switched Telephone Network (“PSTN”) from any place inside the coverage area of the cellular wireless network.
In a typical cellular wireless system, an area is divided geographically into a number of cell sites, each defined by a radio frequency (“RF”) radiation pattern from a respective base transceiver station (“BTS”) antenna. The BTS antennae in the cells are in turn coupled to a base station controller (“BSC”), which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (“MSC”) for instance. The MSC or gateway may then be coupled to a telecommunications network such as the PSTN (public switched telephone network) or the Internet.
When a mobile station (such as a cellular telephone, pager, or appropriately equipped portable computer, for instance) is positioned in a cell, the mobile station communicates via an RF air interface with the BTS antenna of the cell. Consequently, a communication path is established between the mobile station and the telecommunications network, via the air interface, the BTS, the BSC and the MSC.
With the explosive growth in demand for wireless communications, the level of call traffic in most cell sites has increased drastically over recent years. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors (which can be visualized ideally as pie pieces), each defined respectively by radiation patterns from directional antenna components of the respective BTS, or by respective BTS antennae.
In a Code Division Multiple Access (“CDMA”) wireless network, each cell employs one or more carrier frequencies, and each sector is distinguished from adjacent physical sectors by a pseudo-random number offset (“PN offset”). Further, each sector can concurrently communicate on multiple different channels, distinguished by “Walsh codes”. When a mobile station operates in a given sector, communications between the mobile station and the BTS of the sector are carried on a given frequency and are encoded by the sector's PN offset and a given Walsh code.
According to industry standard IS-2000 (published in March 2000 as TIA/EIA/IS-2000-A) and IS-95, a mobile station can communicate with a number of “active” sectors at a time. Depending on the system, the number of active sectors can be up to three or six (currently). The mobile station receives largely the same signal from each of the active sectors and, on a frame-by-frame basis, selects the best signal to use.
A mobile station maintains in its memory a list of the sectors in its “active” set. In addition, it maintains in its memory a list of “candidate” sectors (up to six), which are those sectors that are not yet in the active set but that have sufficient signal strength that the mobile station could demodulate signals from those sectors. Further, the mobile maintains a list of “neighbor” sectors, which are those sectors not in the active set or candidate set but are in close vicinity to the mobile station. All other possible sectors are members of a “remaining” set.
In existing systems, to facilitate a determination of which sectors should be in the mobile station's “active” set, all base stations emit a pilot channel signal, typically at a power level higher than other downlink signals. A mobile station then constantly measures the strength (Ec/Io, i.e., energy versus spectral density) of each pilot that it receives and notifies a primary base station (a base station currently serving the mobile station) when pilot strength falls above or below designated thresholds. The base station, in turn, provides the mobile station with an updated list of active pilots.
More particularly, according to IS-2000, the base station initially provides the mobile station with a Handoff Direction Message (HDM), which indicates (i) the PN offsets of the sectors in the active set and (ii) the following handoff parameters that relate to pilot signal strength:                T_ADD: Threshold pilot strength for addition to the active set (e.g., −14 dB)        T_COMP: Difference in signal strength from an active set pilot (e.g., 2 dB)        T_DROP: Threshold pilot strength for removal from the active set (e.g., −16 dB)        T_TDROP: Time for which an active set pilot falls below T_DROP to justify removal from the active set (e.g., 2 seconds)Additionally, the base station initially provides the mobile station with a Neighbor List Update Message (NLUM), which identifies the “neighbor” sectors for the current active set.        
The mobile station then monitors all of the pilot signals that it receives, and the mobile station determines if any neighbor pilot exceeds T_ADD by T_COMP. If so, the mobile station adds the pilot to its “candidate” set and sends a Pilot Strength Measurement Message (PSMM) to the base station, indicating the estimated Ec/Io, for the pilot. Depending on current capacity and other issues, the base station may then send an HDM to the mobile station, listing the pilot as a new member of the active set. Upon receipt of the HDM, the mobile station then adds the pilot to its active set as instructed, and the mobile station sends a Handoff Completion Message (HCM) to the base station, acknowledging the instruction, and providing a list of the pilots (PN offsets) in its active set.
Similarly, if the mobile station detects that the signal strength of a pilot in its active set drops below T_DROP, the mobile station starts a handoff drop timer. If T_TDROP passes, the mobile station then sends a PSMM to the base station, indicating the Ec/Io and drop timer. The base station may then respond by sending an HDM to the mobile station, without the pilot in the active set. The mobile station would then receive the HDM and responsively move the pilot to its neighbor set and send an HCM to the base station.